It is known to use gravity borehole tools to measure characteristics of geologic formation, particularly in the exploitation of hydrocarbon reservoirs found in geologic formations or in the subsurface storage of carbon dioxide or water. Exploitation of hydrocarbon reservoirs involves characterizing oil, gas, and/or water content of subterranean formations.
The process of measuring physical properties of earth formations beneath the surface of the earth is commonly referred to as “well logging”. It comprises the step of lowering sensors or testing equipment mounted on robust tool bodies into a wellbore drilled through the earth. When the tool is suspended from an armored cable the process is more specifically referred to as “wireline” well logging. Alternative conveyance techniques as known in the art include lowering the instruments mounted on drill pipe, casing or production tubing or on coiled tubing. The drill pipe conveyance technique, in particular, is known as “logging while drilling” when measurements are performed during the actual drilling of a wellbore.
Specifically, borehole gravity measurements are a direct measure of the bulk density of the formation surrounding a wellbore. Typically gravity data are taken at different vertical depths or stations along the wellbore. The basic principle of borehole gravity measurements is that the change in gravity relates directly to the bulk density contrast of the formation, the distance from the stations and the density contrast body. The bulk density in turn is directly related to grain densities and the pore fluid (gas, oil or water) densities and porosity of the formation.
Several gravity measurement tools are commercially available or have been proposed in the prior art. A commercially available borehole gravity meter (BHGM) is for example manufactured by LaCoste & Romberg of Lafayette, Colo., USA under the trade name “Micro-g system”. Other gravity and gravity difference measuring instruments are described in U.S. Pat. Nos. 5,351,122 and 5,892,151 both issued to Niebauer et al. and 5,903,349 to Vohra et al.
The known gravity tools according to the '151 patent include at least one, preferably several longitudinally spaced apart gravity sensors enclosed in an instrument housing. The gravity sensors are fiber optic interferometry devices, which measure a velocity of a free falling mass by determining, with respect to time, interference fringe frequency of a light beam split between a first path having a length corresponding to the position of the free falling mass, and a second “reference” (fixed length) path. The fringe frequency is related to the velocity of the free falling mass, which can be correlated to earth's gravity by precise measurement of the mass's position and the time from the start of free fall. The measurement of gravity differences is performed by determining a difference in gravity measurements made between two of the individual gravity sensors positioned at locations vertically spaced apart.
Further instruments for gravity and gravity difference measurements are described in the co-owned U.S. Pat. No. 6,671,057 issued to Orban. The proposed instrument includes a gravity sensor with a first mass adapted to fall freely when selectively released from an initial position. The mass has optical elements adapted to change the length of an optical path in response to movements of the mass. The sensor output is coupled to a beam splitter. One output of the splitter is coupled optically directly to an interferometer. Another output of the splitter is coupled to the interferometer through an optical delay line. The frequency of an interference pattern generated is directly related to gravity at the mass. A second such mass having similar optics, optically coupled in series to the first mass and adapted to change the path length in opposed direction when selectively dropped to cause time coincident movement of the two masses, generates an interference pattern having frequency related to gravity difference.
Further known gravity measuring instrument are shown for example in U.S. Pat. No. 7,155,101 to Shah et al.
Methods of applying gravity measuring instruments in the oil industry can be found for example in the above '057 patent and in the U.S. Pat. No. 7,069,780 to Ander, and by J. L. Hare et al. in: The 4-D microgravity method for waterflood surveillance: A model study for the Prudhoe Bay reservoir, Alaska, Geophysics, Vol. 64, No. 1 (January-February 1999), p. 78-87. In the latter study, the gravity observations are inverted to determine the subterranean density distribution. The inversion used in this prior art is posed as a linear, underdetermined inverse problem with an infinite number of possible solutions. The densities range is subjected to a set of constraints resulting in a constrained, linear system which can be solved using least-square methods.
Further forward modeling and inversion techniques are described in the U.S. Pat. No. 6,502,037 to Jorgensen et al and in the U.S. Pat. No. 6,675,097 to Routh et al as well as various other publications including W. R. Green, Inversion of gravity profiles by use of a Backus-Gilbert approach, Geophysics, 40 (1975), 763-772; B. J. Last and Kubik. K., Compact gravity inversion, Geophysics, 48 (1983), 713-721; and Y. Li and Oldenburg, D W., 3D inversion of gravity data, Geophysics, 61 (1996), 2, 394-408.
In geology and reservoir modeling a dip is understood as the angle between a planar feature, such as a sedimentary bed or layer or a fault, and a horizontal plane. A number of different logging tools have been developed and successfully used for many years to determine the dip of the formation beds. The present generation of the resistivity scanner logging tool has for example demonstrated a good ability to estimate the dip angle. These triaxial array induction tools as commercially offered by Schlumberger have the capability to measure at multiple depths of investigation from the wellbore depending on the spacing between transmitters and receivers as described for example by T. Barber et al., Determining Formation Resistivity Anisotropy In The Presence Of Invasion, SPE 90526 (2005). The data are then processed using a 1D inversion algorithm to determine the dip angle of the formations layers. Other logging tools based on micro-resistivity or sonic measurements have also been used to determine the dip of the formation around a wellbore.
In view of the known art, it is seen as an object of the invention to provide a novel method of determining the formation dip and related parameters.